Strong Gusty Crosswinds

Challenging encounters with strong gusty crosswinds during the approach and landing phase in commercial air transport — never routine for flight crews and sometimes underestimated by air traffic control (ATC) — involve some risk because of systemic gaps, mismatches and misconceptions, says Gerard van Es, senior consultant for flight operations and flight safety, National Aerospace Laboratory Netherlands (NLR).

He explained the impetus for further study of the factors involved and a few of NLR’s recently developed recommendations during Flight Safety Foundation’s International Air Safety Seminar in Santiago, Chile, in October 2012. In April, van Es updated AeroSafety World about industry responses to the complete report that he and a colleague, Emmanuel Isambert, prepared as advisers to the European Aviation Safety Agency (EASA).1

Difficult surface wind conditions2 have confronted pilots since the flights of Wilbur and Orville Wright, and one of the many recent examples was a serious incident in Germany in 2008 (see “Serious Incident in 2008 Prompted German and EASA Analyses”) that motivated German accident investigators, and subsequently EASA, to dig deeper into the causal factors and to update mitigations. A German recommendation — calling for assessment of all measuring systems that detect the presence of near-surface gusts and how pilots integrate various wind data into landing/go-around decisions — led to the NLR study for EASA, van Es said.

Serious Incident in 2008 Prompted German and EASA Analyses

Freezing rain caused a two-hour delay in the Airbus A320’s departure from Munich, Germany, for a scheduled flight with 132 passengers and five crewmembers to Hamburg the afternoon of March 1, 2008.

During cruise, the flight crew received a Hamburg automatic terminal information system report of winds from 280 degrees at 23 kt, gusting to 37 kt. They planned for — and later received clearance for — an approach and landing on Runway 23, which is equipped with an instrument landing system (ILS) approach, said the report by the German Federal Bureau of Aircraft Accident Investigation (BFU).

When the crew reported that they were established on the ILS approach, the airport air traffic controller said that the wind was from 300 degrees at 33 kt, gusting to 47 kt.

The report said that a decision to go around would have been reasonable because the controller’s report indicated that the winds exceeded the maximum demonstrated crosswind for landing, which was “33 kt, gusting up to 38 kt” and presented as an operating limitation in the A320 flight crew operating manual.

The captain asked for the current “go-around rate,” and the controller replied, “Fifty percent in the last 10 minutes.” The controller offered to vector the aircraft for a localizer approach to Runway 33, but the captain replied that they would attempt to land on Runway 23 first.

The crew gained visual contact with the runway at the outer marker. The copilot, the pilot flying, disengaged the autopilot and autothrottles about 940 ft above the ground. She used the wings-level, or crabbed, crosswind-correction technique until the aircraft crossed the runway threshold and then applied left rudder and right sidestick to decrab the aircraft — that is, to align the fuselage with the runway centerline while countering the right crosswind.

The A320 was in a 4-degree left bank when it touched down on the left main landing gear and bounced. Although the copilot applied full-right sidestick and right rudder, the aircraft unexpectedly rolled into a 23-degree left bank. It touched down on the left main landing gear again, striking the left wing tip on the runway, and bounced a second time.

The crew conducted a go-around and landed the aircraft without further incident on Runway 33. The left wing tip, the outboard leading-edge slat and slat rail guides were found to have been slightly damaged during the serious incident, the report said, but the ground contact was not detected by the flight crew.

The BFU, in its final report, listed the immediate causes: “The sudden left wing down attitude was not expected by the crew during the landing and resulted in contact between the wing tip and the ground. During the final approach to land, the tower reported the wind as gusting up to 47 kt, and the aircraft continued the approach. In view of the maximum crosswind demonstrated for landing, a go-around would have been reasonable. System-level causes were: “The terminology maximum crosswind demonstrated for landing [italics added] was not defined in the Operating Manual (OM/A) and in the Flight Crew Operating Manual (FCOM), Vol. 3, and the description given was misleading. The recommended crosswind landing technique was not clearly described in the aircraft standard documentation. The limited effect of lateral control was unknown.”

In the relevant time period, the surface wind at Hamburg was being measured by German Meteorological Service anemometers located near the thresholds of Runways 23/33 and 15, and was logged at 10-second intervals. Air traffic controllers also had data on maximum veer angle and peak wind speed for the preceding 10 minutes. “In the final 10 minutes prior to the occurrence, the wind direction varied between 268 degrees (minimum) and 323 degrees (maximum),” the report said. “In this period, the maximum gust speed recorded was 47 kt [Figure 1].”

When the controller later gave the crew clearance to land on Runway 33, the information included wind from 300 degrees at 33 kt gusting to 50 kt (two-minute mean value). Four additional wind reports were issued to the crew before touchdown, the final one for wind from 290 degrees at 27 kt gusting to 49 kt.

“The investigation showed that wing tip contact with the runway was not due to a single human error, a malfunction of the aircraft or inadequate organisation; rather, it was due to a combination of several factors,” the report said, citing the automatic transition from lateral flight mode to lateral ground mode control laws when the left gear first touched down, resulting in half of full travel in response to full side­stick deflection.

“The fact that there were no significant gusts during the decrab procedure explains that the aircraft was not brought to this unusual and critical attitude by direct external influence. … The BFU is of the opinion that the captain as pilot-in-­command did not reach his decision using … reasoning [regarding lower crosswind component on Runway 33], because he did not regard the value maximum crosswind demonstrated for landing as an operational limit for the aircraft. Civil air transport pilots were generally poorly informed about the effects of crosswinds in weather conditions such as these.”

During this investigation, 81 pilots holding air transport pilot licenses and employed by five different airlines provided anonymous survey responses in which they were about evenly divided in understanding maximum demonstrated crosswind as a guide versus a limit. Significant differences in understanding also were found concerning the practical application of maximum demonstrated crosswind.

The serious incident involving the Airbus A320-211 at Hamburg on March 1, 2008, and related events were analyzed and safety recommendations about landing in strong gusty crosswind conditions were issued by the German Federal Bureau of Aircraft Accident Investigation in Investigation Report 5X003-0/08, March 2010.

— Mark Lacagnina and Wayne Rosenkrans

Crosswind-related regulations originated in a period from a few years after World War II to 1978, when demonstrated crosswind in airworthiness-certification regulations became fixed for industry use, van Es said.3

NLR’s scope included querying operators about understanding of aircraft certification for crosswind and relevant policies and procedures; a brief review of factors in crosswind-related occurrences; a review of measurement technologies; and the salience of wind instrument precision.

“First of all, we noticed that the way of arriving at and presenting the [crosswind] information varies between the manufacturers and even between the aircraft models,” van Es said. “Most [manufacturers] don’t mention any kind of gusts, but also the way they’ve derived the [demonstrated crosswind value] during the flight test can be very different, giving different results. And they are allowed to, and the regulations on the means of compliance [allow them] this opportunity. Limits, real hard limits, are very rare, nor are they required to be established. Typically, it’s up to the operators to decide if they transfer a demonstrated value into a hard limit. … This all can result in a possible mismatch [between] what the operator is using and what the data from the manufacturer is telling [us].”

The NLR survey was sent to 115 operators from Asia, Europe and North America, and yielded 36 operator responses. “Basically they were telling a story that we were expecting, to some extent,” van Es said, especially regarding the variability in practices. “They were very keen to see what others were doing and what the issues were,” given their anecdotal knowledge of many crosswind-related occurrences.4

Wind Data Sources

Operators and pilots have several disadvantages as they integrate complex factors. “First of all, there is no common interpretation of the manufacturer’s crosswind,” he said. “[Respondents] operate similar models, and they have a different view of what was told to them or what was written in the manuals provided to them. When it came to reported gust values in their operation — the wind reports, how to deal with gusts — some operators said, ‘We don’t take into account the gusts when we look at the reported wind values.’ Others said, ‘Yes, we do, and we do it this way.’ Others said, ‘We do, but we don’t specify how to deal with the gusts.’”

Each type of wind information has advantages and limitations. “FMS [flight management system–derived] wind is something that you have to be very careful in using, especially during the approach,” van Es said. “[Yet] some operators … said use of FMS wind is encouraged and [indicates] good airmanship. Others said, ‘It’s strictly prohibited because we had incidents where we nearly lost the aircraft by using FMS winds.’” Problems in relying on this source in this context include lack of system correction for side slip, its use of an average value and its applicability to winds at altitude — not at the surface.

Some respondents’ pilots request from ATC a series of instantaneous wind reports during approach. “These are snapshots — the actual [real-time] wind that is available as measured at the airport,” he said. “Typically, you get an average [two-minute] wind, but some airports allow you to ask for an instantaneous wind [report].” Some respondents promote the use of instantaneous winds; overall, there was no common way of determining the components either in tailwind or in crosswind.

The survey also found that 75 percent of respondents use a combination of demonstrated and advised crosswinds, and a number of these set maximum crosswind values lower than the manufacturer’s demonstrated/advised crosswinds; 82.9 percent use the crosswind values as hard limits; 67 percent have procedures for how their pilots should calculate the crosswind component, with 58 percent of these specifying how the pilots should take gusts into account; and 33 percent do not include gusts in their crosswind values. “A small number of the respondents left the decision — to include gusts or not — up to the captain,” the report said.

Risk of Confusion

NLR researchers usually found that in occurrence reports, only the wind data reported on the automatic terminal information service (ATIS) had been considered by the flight crew in preparing for an approach, while all respondents cited control tower wind reports as their primary source. “So the reported wind that they got just before landing was not taken into account [in the occurrence reports],” van Es said. “And what happened in the 30 minutes that [elapsed as they] were planning the approach [was that by] the actual landing, the wind had changed. That happens all the time; the wind encountered is completely different from what is reported. They got a much stronger wind.”

Frequently in cases selected, the pilot flying used an incorrect crosswind technique, not following the manufacturer’s recommendation. Even low-velocity crosswind/gusts can be very difficult if the flight crew fails to correctly apply the procedure.

Figure 1 from the NLR work gives a sense of the pilots’ expectations versus the reality they encountered in comparable models/types of large commercial jets. “For several cases — excursions, hard landing, tail strikes, wing/pod strikes — what we see is that more than half of these occurrences [take place in crosswind conditions that are less than] what was demonstrated,” he said.

The two most prevalent wind sensors approved for airport runways with accurate gust-measurement capability are the cup/propeller type with a wind vane, and the ultrasonic type (often called sonic type). Both measure data within 2 to 4 percent of the correct value.

“The normal [ATIS/control tower] wind report that you get is an average,” van Es said. “It is a forecast of the wind that you’re supposed to expect. Many pilots think it is an actual [real-time] measurement; it is not. It is a two-minute average, and they came up with this [to provide users] a good balance between the mean error and the absolute error in the forecast.”

The NLR report published by EASA includes a list of recommended mitigations for the issues identified, and van Es discussed some examples. “First of all … include gusts when decomposing reported wind into the crosswind component and take the gust component [as] fully perpendicular to the runway,” he said. In the United States in the 1950s and 1960s, this practice was mandatory, NLR found. Flight crews always should use the most recent wind report in decision making.

Despite the willingness of controllers to provide a series of instantaneous wind reports on request during an approach involving strong gusty crosswinds, NLR researchers advise against using this source. “[In] several incidents … the pilot was asking for … the instantaneous wind every 10 seconds,” he said. “And [these values] went all over the place until [one was] below his company limit, and then he said, ‘Yeah, going to land.’ He went off [the runway].”

As noted, applying the manufacturer’s crosswind-handling technique for the specific aircraft type/model/size is the best practice in risk management. But even this cannot be 100 percent successful, given the unique and dynamic forces in play. “The poor pilot … is confronted with all kinds of confusion and issues when he has to decide whether or not to land in a gusty crosswind,” van Es said. “It should be company policy that you can ask for another runway or divert if you don’t feel comfortable — if the wind conditions are unfavorable — because that is a very good defense in these cases.”

Since the release of the 2010 and 2012 reports, with further EASA–NLR communication through industry forums and pending articles for airlines’ safety magazines, a number of operators say they will revisit their policies and procedures, van Es told AeroSafety World. Convincing civil aviation authorities, however, is likely to take more time.

“The regulatory [part] is always difficult in terms of who is taking the lead in this case, especially because it’s a multi-actor issue,” he said, and this involves the initiative of operators, manufacturers, regulators and the aviation meteorology community. “The regulators are hesitating to go left or right. They don’t know exactly what to do.”

Basically, the problem they face is some degree of mismatch in certification of aircraft versus operational use of aircraft. “What EASA has said is that they are looking to publish … a sort of safety bulletin on this topic,” van Es said. “But changing regulations? I think that’s a step too far for them. There are big advantages in educating the pilots because they often have great difficulties in understanding … wind report [sources]. There is a lot of misconception within crews about how the systems work. … The best experience is the real experience, but for an average line pilot, to have a lot of these landings could be quite rare.”

Citing World Meteorological Organization (WMO) WMO-No. 731, the NLR report published by EASA says, “A gust can be defined as the difference between the extreme value and the average value of the wind speed in a given time interval. A gusty wind is characterized by rapid fluctuations in wind direction and speed. At airports, gustiness is specified by the extreme values of wind direction and speed between which the wind has varied during the last 10 minutes.”

For example, EASA’s internationally harmonized regulation (Part 25.237, “Wind Velocities”) states, “For landplanes and amphibians, a 90-degree cross component of wind velocity, demonstrated to be safe for takeoff and landing, must be established for dry runways and must be at least 20 kt or 0.2 VSO, whichever is greater, except that it need not exceed 25 kt. Note that VSO means the stall speed or the minimum steady flight speed in the landing configuration.”

The report said, “Since 1990, there have been more than 280 approach and landing [accidents] and 66 takeoff accidents/incidents investigated with [Part] 25–certified aircraft operated in commercial operations worldwide in which crosswind or tailwind was a causal factor. Occurrences related to gusty wind conditions are also very common in Europe. … The wind in these occurrences was often very gusty.”